Concern over the emission of acid rain precursors, NO.sub.x and SO.sub.x, from stationary combustion processes is likely to lead to increasingly tight regulations. In certain areas, no single method of in-furnace or post-combustion reduction of these pollutants suffices. For reducing NO.sub.x emission, methods of post-combustion flue gas treatment include the injection of ammonia in the presence of a sufficient amount of O.sub.2 to selectively reduce NO.sub.x from the combustion effluent. Reduction of SO.sub.2 may be affected by injection of calcium compounds.